Split piston for an internal combustion engine

ABSTRACT

The invention relates to a split piston for an internal combustion engine, comprising an upper piece, connected to a lower piece, by means of a screw connection. The screw connection comprises a tubular connector piece, mounted on the underside of the piston upper piece with an external thread, a spacer sleeve with an external and an internal thread and a support rib formed on the lower piece of the piston with an internal thread. The upper piece can thus be screwed to the lower piece, by means of the connector piece, the spacer sleeve and the support rib. The spacer sleeve acts as a compressing body and the connector piece as an extending body, to generate the mechanism tension necessary for fixing the screw connection.

CROSS REFERENCE TO RELATED APPLICATIONS

Applicant claims priority under 35 U.S.C. §119 of German Application No.103 37 961.4 filed Aug. 19, 2003. Applicant also claims priority under35 U.S.C. §365 of PCT/DE2004/001852 filed Aug. 19, 2004. Theinternational application under PCT article 21(2) was not published inEnglish.

The invention relates to a multi-part piston for an internal combustionengine, in accordance with the preamble of claim 1.

A multi-part piston for an internal combustion engine is known from theGerman patent 969 838, which consists of a main piston body and a crownpart connected with it by means of expansion screws. In order toguarantee a secure screw connection even at faster back and forthmovements of the piston, a greater number of expansion screws arerequired for assembly of the crown part to the main piston body. Thishas the disadvantages that assembly of the known piston is verytime-consuming and labor-intensive.

It is the task of the invention to avoid these disadvantages of thestate of the art. This task is accomplished with the characteristicsaccording to the characterizing part of the main claim. Practicalembodiments of the invention are the object of the dependent claims.

The advantages of quick, easy, and reliable assembly of the pistonaccording to the invention are achieved in that here, an intermediatesleeve is first screwed into a ring rib of the lower part of the piston,after which the entire upper part of the piston is screwed into theintermediate sleeve. In this connection, the intermediate sleeve acts asa compressing body that produces a mechanical stress that serves to fixthe screw connection in place.

The invention is described below, using the drawings. These show:

FIG. 1 a two-part piston in cross-section, the upper part of which isscrewed to its lower part by way of an intermediate sleeve, and

FIG. 2 an enlarged cross-sectional view of a detail of the piston, withthe intermediate sleeve according to the invention.

FIG. 1 shows a multi-part cooled piston 1, which consists of an upperpart 2 and a lower part 3. The upper part 2 and the lower part 3 aremade of steel. The piston crown 4, the inner region of which has acombustion bowl 5, is formed by the upper part 2. A ring wall 6 ismolded onto the outer edge of the piston crown 4, the outer surface ofwhich wall forms the top land 7, on the piston crown side, which land isfollowed by the ring belt 8 having ring grooves for accommodating pistonrings, not shown in the figure.

Two pin bosses 17, 17′ each having a pin bore 18, 18′, respectively, aredisposed on the side of the lower part 3 that faces away from the pistoncrown 4, the faces 19, 19′ of which are disposed to be set back relativeto the ring wall 6, in the direction of the piston longitudinal axis 20.Piston skirt elements 21 are connected with the pin bosses 17, 17′ andthe lower part 3 of the piston 1.

The upper part 2 and the lower part 3 of the piston 1 are connected withone another, for one thing, by way of an inner contact region 9 and, foranother, by way of an outer contact region 10 that is disposedconcentric to the former. The inner contact region 9 is formed by acontact surface 11 that is disposed on the side of the upper part 2 thatfaces away from the combustion bowl 5, and by a contact surface 13 thatis disposed on a ring-shaped support rib 12 of the lower part 3, on thepiston crown side. The outer contact region 10 is formed by a contactsurface 14 that delimits the underside of the ring wall 6, and by acontact surface 16 that is disposed on a ring-shaped support land 15 ofthe lower part 3, on the piston crown side.

A cooling channel 22, disposed in the edge region of the piston crown 4,is formed by upper part 2 and lower part 3 of the piston 1, the radiallyouter delimitation of which channel is formed by the ring wall 6, itsradially inner delimitation partly by the support rib 12 and partly bythe piston crown 4 that delimits the combustion bowl 5, and its axiallylower delimitation by the lower part 3 of the piston 1. Cooling oil ispassed into the cooling channel 22 and passed out of the cooling channel22 by way of inlet and outlet openings not shown in the figure.

During the assembly of upper part 2 and lower part 3, the coaxialalignment of these two parts is achieved in that the support land 15 isformed in step shape in cross-section, and the lower face of the ringwall 6 has a cylinder-shaped recess 23 on the inside, the inside shapeof which corresponds to the outside shape of the support land 15, sothat during assembly of the upper and lower part 2, 3 of the piston 1,the support land 15 can be introduced into the recess 23 and thereby theupper and lower part 2, 3 can be aligned coaxially.

On the side facing away from the combustion bowl 5, the piston crown 4has a tube-shaped connection projection 24 disposed coaxial to thepiston longitudinal axis 20, the length of which does not exceed thedistance from the piston crown 4 to the pin bosses 17, 17′. The outsideof the end of the connection projection 24 that faces away from thepiston crown 4 is provided with an outside thread 25. Furthermore, theinside of the support rib 12 has an inside thread 26 close to the pistoncrown 4. In this connection, the radii of the support rib 12 and theconnection projection 24 are selected in such a manner that the radialdiameter of the inside thread 26 of the support rib 12 is greater thanthe radial diameter of the outside thread 25 of the connectionprojection 24. Furthermore, the axial distance between the outsidethread 25 and the pin bosses 17, 17′is less than the axial distancebetween the inside thread 26 and the pin bosses 17, 17′.

In order to screw the upper part 2 and the lower part 3 together withone another by way of the two threads 25 and 26, a tube-shapedintermediate sleeve 27 is therefore required, the length of whichcorresponds to the difference in the distance between the outside thread25 and the pin bosses 17, 17′ and the distance between the inside thread26 and the pin bosses 17, 17′, and which has an inside thread 28 and anoutside thread 29 at least at its ends, whereby the thread 28 that lieson the inside fits onto the outside thread 25 of the connectionprojection 24, and the thread 29 that lies on the outside fits onto theinside thread 26 of the support rib 12. Because the intermediate sleeve27 comes into contact with the connection projection 24 and the supportrib 12 only in the region of the thread connections 25, 26, 28, 29, itis guaranteed that during assembly of upper and lower part 2, 3 of thepiston 1, the connection projection 24 experiences expansion, and theintermediate sleeve 27 experiences compression, by means of whichdeformations mechanical stress that serves to fix the connections isexerted on the thread connections between upper part and lower part 2, 3of the piston 1. So that the deformations of the connection projection24 and the intermediate sleeve 27 that serve to fix the threadconnections are sufficiently great, the wall thickness of these partsmust be dimensioned accordingly.

In the exemplary embodiment according to the figure, the connectionprojection 24 has a screw flange 30 with the outside thread 25 at itsend facing the pin bosses 17, 17′, the thread 28 that lies on the insideis disposed on the entire inside of the intermediate sleeve 27, and theend of the intermediate sleeve 27 that faces the piston crown 4 also hasa screw flange 31 with the thread 29 that lies on the outside. In thisway, the conditions required for deformation of the intermediate sleeve27 and the connection projection 24, as described above, are met.

FIG. 2 shows an enlarged view of the screw connection between the upperpart 2 and the lower part 3 of the piston 1, which takes place by way ofthe intermediate sleeve 27. For this purpose, the connection projection24 has the screw flange 30 on its end facing away from the piston crown4, which flange carries the outside thread 25 on its circumferencesurface that lies radially on the outside. The intermediate sleeve 27has the thread 28 on its entire surface that lies radially on theinside, onto which the outside thread 25 of the screw flange 30 fits. Onits end facing the piston crown 4, the intermediate sleeve 27 also has ascrew flange 31, which carries the thread 29 on its circumferencesurface that lies radially on the outside, which thread fits onto theinside thread 26 that is disposed on a surface of the support rib 12that lies radially on the inside and borders on the upper part 2. Inthis connection, the face 32 of the intermediate sleeve 27 that lies onthe piston crown side forms a common, level contact surface for theupper part 2 of the piston 1, with the contact surface 13 disposed onthe support rib 12.

Assembly of the piston 1 takes place in simple manner, in that first theintermediate sleeve 27 is screwed into the ring rib 12 of the lower part3, and subsequently the upper part 2 of the piston 1 is screwed into thethread 28 of the intermediate sleeve 27, which lies on the inside, byway of the outside thread 25 of the connection piece 24. When the upperpart 2 and the lower part 3 are screwed together, the lower contactsurface 11 of the upper part 2 first comes into contact with the contactsurface 13, 32 formed by the support rib 12 and the intermediate sleeve27, and afterwards, further tightening of the upper part 2 on the lowerpart 3 results in the effect that the connection projection 24experiences expansion in the direction of the arrow 33, and theintermediate sleeve 27 experiences compression in the direction of thearrow 34. Because of these deformations, mechanical stress is exerted,in particular, on the thread connection 25, 28 between the connectionprojection 24 and the intermediate sleeve 27, and this makes asignificant contribution to fixation of this thread connection 25, 28.

REFERENCE SYMBOL LIST

-   1 piston-   2 upper part of piston 1-   3 lower part of piston 1-   4 piston crown-   5 combustion bowl-   6 ring wall-   7 top land-   8 ring belt-   9 inner contact region-   10 outer contact region-   11 contact surface-   12 support rib-   13,14 contact surface-   15 support land-   16 contact surface-   17, 17′ pin boss-   18, 18′ pin bore-   19, 19′ faces of the pin bores 18, 18′-   20 piston longitudinal axis-   21 piston skirt element-   22 cooling channel-   23 recess-   24 connection projection-   25 outside thread-   26 inside thread-   27 intermediate sleeve-   28, 29 thread of the intermediate sleeve 27-   20, 31 screw flange-   32 face of the intermediate sleeve 27-   33, 34 arrow

1. Multi-part piston (1) for an internal combustion engine having anupper part (2) that has a piston crown (4), on which part a ring wall(6) having a ring belt (8) is molded on, and having a lower part (3)that is connected with the upper part (2) by way of a screw connection(12, 24-31) and has pin bosses (17, 17′) having pin bores (18, 18′) onits lower side facing away from the upper part (2), and having pistonskirt elements (21) connected with the pin bosses (17, 17′), wherein thescrew connection consists of the following elements: of an essentiallytubeshaped connection projection (24) that is affixed on the undersideof the piston crown (4) and is disposed coaxial to the pistonlongitudinal axis (20), and has an outside thread (25) on its end thatfaces away from the piston crown (4) in the axial direction, said threadbeing disposed on a circumferential side of the connection projection(24) that lies radially on the outside of the connection projection, ofa support rib (12), said support rib being molded onto the lower part(3), said rib having an inside thread (26) on its piston crown end,viewed in the axial direction, said thread being disposed on a surfacethat lies radially on the inside of said rib, and of an essentiallytube-shaped intermediate sleeve (27) having a thread (28) that lies onthe inside and a thread (29) that lies on the outside, by means of whichthe upper part (2) can be screwed together with the lower part (3), byway of the connection projection (24), and by way of the support rib(12), and which sleeve is configured in such a manner that when thishappens, the connection projection (24) can be expanded and theintermediate sleeve (27) can be compressed.
 2. Piston according to claim1, wherein the thread (29) that lies on the outside is disposed on acircumference surface of the piston crown end of the intermediate sleeve(27), which surface lies radially on the outside, and fits into theinside thread (26) of the support rib (12), and that the thread (28)that lies on the inside is disposed on a surface of the intermediatesleeve (27), which surface lies radially on the inside, and fits ontothe outside thread (25) of the connection projection (24).
 3. Pistonaccording to claim 2, wherein the axial distance between the insidethread (26) of the support rib (12) and the piston crown (4) is lessthan the axial distance between the outside thread (25) of theconnection projection (24) and the piston crown (4), and that the radialdiameter of the inside thread (26) is greater than the radial diameterof the outside thread (25).
 4. Piston according to claim 1, wherein thewall thickness of the connection projection (24) and that of theintermediate sleeve (27) are dimensioned in such a manner that when theupper part (2) is screwed together with the lower part (3), by way ofthe connection projection (24) and by way of the intermediate sleeve(27), the connection projection (24) can be expanded to such an extent,and the intermediate sleeve (27) can be compressed to such an extentthat in this way, a mechanical stress that serves to fix the screwconnection (12, 24-31) in place can be generated.
 5. Piston according toclaim 1, wherein the connection projection (24) has a screw flange (30)having the outside thread (25), on its end facing away from the pistoncrown (4), that the thread (28) that lies on the inside is disposed onthe entire inside of the intermediate sleeve (27), and that theintermediate sleeve (27) has a screw flange (31) having the thread (29)that lies on the outside, on its end that faces the piston crown (4).